As the development of offshore fields move into deeper waters the requirements on the vessel mooring system become more challenging. In many areas a turret mooring system is used to moor the vessel at the offshore location.
The use of taut mooring line systems is becoming increasingly widespread for such offshore installations. In these mooring systems the mooring line is held under a tension which results in the line being fully suspended above the seabed. The mooring system relies on the elasticity in the mooring lines to accommodate vessel movements and loads rather than on the weight of the catenary in the suspended mooring lines. The mooring lines are thus loaded continuously in a dominant direction.
This results in the mooring lines approaching the vessel within a very limited range of approach angles at the mooring line chain stoppers. It also results in many small fluctuations in the mooring line angles resulting in fatigue stresses in the chains if typical single axis chain stoppers are used. These are typically referred to as out of plane bending stresses in the chain links.
To address this fatigue problem chain stopper assemblies with a trunnion block pivotally supported by the turret structure with a horizontal pivoting axis, and a chain stopper coupled to the trunnion block with a pivoting axis perpendicular to the horizontal have been proposed, for example in US 2010/0175604 and U.S. Pat. No. 7,325,508. Such chain stopper assemblies allow for movement to accommodate vessel offsets and vessel rolling motion. However, in such chain stopper assemblies the main movement of the chain stopper, which is typically the pivoting around the horizontal axis, acts on the horizontal pivoting axis via the coupling between the chain stopper and the trunnion block, resulting in un-desired loading of the coupling and possible failure of the coupling.
Embodiments of the present invention provide a method of supporting a chain stopper on a vessel, a chain stopper assembly for a vessel, and a vessel that seek to address at least one of the above problems.